The objective of a typical terrestrial-based location service for mobile devices is to infer the location of a client device at a given instance of time relative to the known locations of a set of network beacons. Wi-Fi positioning system (WPS) can provide location in certain situations (such as indoors) by taking advantage of the rapid growth of wireless access points (WAPs) as beacons in urban areas. A provider of this type of service maintains a public database and can determine the position for a device based on the specific access points accessible from the device in each specific location. The localization technique used for positioning with wireless access points is based on measuring the intensity of the received signal (Received Signal Strength or “RSS”) to more uniquely identify each location (usually arranged in a grid comprising a plurality of tiles) using radio frequency (RF) locating methodologies.
However, while it may be generally straightforward and relatively low-cost to implement an RSS-based location service, there are several shortcomings to RSS that limit its accuracy. First, there may be large variations in signal strength at any specific location resulting from electromagnetic interference or multipath propagation of the radio frequency signals. Second, RF propagation is location and environment specific such that two adjacent locations may have very different RF propagation obstacles, and changes in the environment can vary RF signals from moment to moment. Third, RSS measurements can vary based on the orientation of the receiving device and surrounding objects such as human bodies (including the body of the user of the receiving device). In addition, variations in RSS measurements among different device models and even on different devices of the same model can obscure the precision of RSS methods.